In general: don’t do it. If you really really need to, you can try to apply Sjors’ solution.
However, if your app is originally built in the older version, and important enough to warrant the effort of a downgrade, you probably have the teamserver enabled. Using that, you should be able to go back to any previous version of your application. For example, you can create a branch from a revision/commit that still had the old Mendix version.
With the ‘mprtool.exe’ in your Studio Pro folder you can change the metadata of a project, for instance the version.
This should be done with great caution and could seriously break stuff if you have components which are not compatible with an older version. Beware..